This application claims the entire benefit of Japanese Patent Application No. 2010-113456 filed on May 17, 2010 the entirety of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a cooling structure for a machine tool main spindle supported by a plurality of bearings so as to be rotatable, and more particularly to a cooling structure for a main spindle that rotates at a high speed, such as for a machining center, for example.
2. Description of Related Art
In recent years, the rotational speed of a main spindle of a machine tool, in particular a main spindle of a machining center, has become higher. Therefore, the amount of heat produced by bearings is becoming larger, which adversely affects the thermal stability of the main spindle and the tool, and so forth. Against such a background, it is necessary to cool the bearings from both the inner race side and the outer race side in order to enhance the thermal stability of the tool and suppress an increase in surface pressure due to the difference between the temperatures of the inner race and the outer race of the bearings. Thus, cooling structures in which the main spindle is directly cooled to cool the inner race side of the bearings in order to reduce the difference between the temperatures of the inner race and the outer race of the bearings have been developed.
In some cooling structures according to the related art in which coolant is introduced into the main spindle, a centrifugal force due to rotation of the main spindle is utilized. For example, Japanese Examined Patent Application Publication Number H07-106534 discloses a configuration in which coolant is delivered to a coolant supply passage formed along the axis of a drawbar to cool a rotor and an axial center of the drawbar and then be recovered. Japanese Patent Application Publication Number H07-9300 discloses a structure in which coolant is supplied from an intermediate location between a front bearing and a rotor and in which a flow path extending to the radially outer side of a main spindle is formed.
Meanwhile, Japanese Patent Application Publication Number H09-317778 discloses a configuration in which a centrifugal force is not utilized. In the configuration, a flow path extending from the radially outer side of a main spindle to the radially inner side of the main spindle, then extending in the longitudinal direction under a bearing, and extending to the radially outer side is formed to cool the inner race of the bearing for the main spindle.
In the technology disclosed in Japanese Examined Patent Application Publication Number H07-106534 described above, however, it is necessary to supply the coolant from the rear end of the main spindle with a small diameter, which imposes limitations on the location of supply of the coolant. In addition, the flow path is longer than necessary, which increases the number of constituent parts and leads to imbalance. The technology disclosed in Japanese Patent Application Publication Number H07-9300 uses a nozzle to supply the coolant from the radially inner side of the main spindle. This increases the number of constituent parts, which leads to a cost increase. In addition, the use of the nozzle does not allow the coolant to be efficiently fed into the main spindle, which deteriorates the cooling efficiency.
The technology disclosed in Japanese Patent Application Publication Number H09-317778 is advantageous in terms of the cost and the degree of freedom in design because the coolant flows only through a desired portion to be cooled. However, the coolant flows through a hole-shaped flow path extending from the radially outer side to the radially inner side of the main spindle. Thus, when the main spindle is rotating at a high speed, the coolant is subjected to a large centrifugal force, and it is necessary to apply a high pressure to the coolant in order to cause the coolant to flow to the radially inner side. Therefore, a seal structure that prevents leakage of the coolant is required, which complicates the cooling structure.